INTERNATIONAL JOURNAL OF MULTIDISCIPLINARY SCIENCES AND ENGINEERING, VOL. 5, NO. 10, OCTOBER 2014 Austempering Heat Treatment of AISI 4340 Steel and Comparative Analysis of Various Physical Properties at Different Parameters

Najeeb Niazi1,*, Salman Nisar1 and Aqueel Shah1 1Department of Manufacturing Engineering and Management, National University of Science & Technology PNEC, Karachi, Pakistan *[email protected]

Abstract— In this study a special heat treatment process named  To improve wear resistance of materials. austempering on AISI 4340 steel is carried out. Heat treatment on steel is carried out to enhance mechanical properties. In this  To reduce the surface cracking and distortion during regard, it is considered essential to undertake a study to heat treatment evaluate different changes occurred in AISI 4340 steel in terms Austempered steels have many industrial applications due of hardness, tensile strength and impact strength at different to improved mechanical properties. Austempering is a austempering temperatures and cooling times and achieving the nontraditional heat treatment technique in which bainite is best combination of these improved mechanical properties for obtained in microstructure of steel. In austempering process to better and optimum utilization of this grade of steel. By using software Design Expert DOE is formulated with Taguchi avoid the formation of pearlite or martensite, the steel is orthogonal arrays comprising of L18 (3*3) with 03 factors and quenched in a bath of molten metal’s or salts. This quickly 03 responses to be calculated. Results of experiments are cools the steel and let it pass away that point where pearlite analyzed via Taguchi method. Signal to noise ratio of responses can be formed resulting into achieving the bainite-forming are carried out to determine the significant factors among the range. The steel is then held at the bainite-forming 03 factors chosen for experimental runs. Overall analysis temperature in the molten salt bath till bainite structure fully showed that impact factor along with hardness is improved to forms. The steel is then taken out of salt bath and allowed to great extent by austempering process. cool in air. Austempering process can produce either upper or lower bainite depending on temperature rate [5]. For T ~ 300- Keywords— Austempering Temperature, AISI 4340 Steel, Bainite and TAGUCHI 540°C upper bainite will be formed. For T ~ 200-300°C, lower bainite will be formed. The austempering cycle will comprise of following steps [6]-[7]: I. INTRODUCTION • Heating the steel to a temperature within the austenitizing range, usually 790 to 915 °C for one hour. ISI 4340 is a medium carbon steel with a carbon content • Quenching in a molten salt bath of NaNO3 maintained at a A ranging from 0.38 to 0.43 % Carbon and usually referred constant temperature, usually in the range of 260 to 400 as nickel - chromium – molybdenum high tensile steel. °C. AISI 4340 are used widely in aircraft, particularly in critical applications such as landing gears, as a structural steel for • Allowing the steel to transform isothermally to bainite in aircrafts and also in automobiles (military as well as this bath for certain time duration. commercial), aerospace equipments, shafts, crank shafts, • Cooling it to room temperature, usually in still air. connecting rods, heavy duty axels etc [1]. Steels can be heat treated to high hardness and strength levels by using variety of methods [2]. Structural components of aircrafts, automobiles etc subjected to high operating stress need the high strength and need resist wear and deformation. Austempering of steel has a wide range of industrial application and can be well substituted by conventional quench and temper process due to following major advantages [3]-[4].  To enhance mechanical properties of hardness.  To enhance and improve impact strength and ductility Fig. 1: Schematic diagram for austempering process of materials.  Improve resistance against subsequent embrittlement.

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o II. EXPERIMENTAL PROCEDURES An average value of 300 C for Ms (Martensite start temperature) of AISI 4340 samples is obtained. After The material used for study is AISI 4340 steel. Three types determination of Ms, the temperature range for austempering of mechanical testing are required to be carried upon samples temperature can be determined and chosen for salt bath of AISI 4340 which are austempered. For each test a specific (quenching medium) temperature in which the samples will shape and size of specimen is to be prepared. The tests be quenched. 03 temperature ranges above the Ms include impact testing, tensile testing and hardness testing. Temperature i.e., 320, 350, 380 oC are considered. Samples of AISI 4340 purchased from local market are in raw form of metallic bars and rods. The bars and rods are B) Samples Preparation converted to specified samples as per required dimensions Samples of AISI 4340 purchased from local market are in according to ASTM A370 by undergoing it through machining raw form of metallic bars and rods. The bars and rods are process of lathe and finishing them in end. The spectro converted to specified samples as per required dimensions graphic analysis of AISI 4340 steel was conducted in the start according to ASTM A370 by undergoing it through machining of study to analyze the chemical composition of steel being process of lathe and finishing them in end. The exact used the result showed following composition: dimension and configuration of samples are mentioned below

for impact testing and tensile testing. Chemical composition (Wt %) C Si Mn P S Cr Mo Ni 0.42 0.2 0.6 0.035 0.04 0.75 0.25 1.8

A) Ms (Martensite Start Temperature) for AISI 4340 Tensile test specimen Determination of Ms or Martensite start temperature for a certain grade of steel prior undergoing heat treatment process on that is very important. A number of techniques and empirical formulae are available in literature to calculate martensite start temperature [8]: We can calculate Ms using formulae. We have used 03 different empirical formulae for calculations and the result is then averaged to get an average Ms Temperature range for our Fig. 2: Samples for tensile test and Impact strength test samples.

Choosing formula of Grange and Stewart: C) Design of Experiments

Ms = 538 – (361×%C) – (39 ×%Mn) – (19 ×%Ni) – (39 Taguchi method is a problem solving tool which can ×%Cr) (in °C) improve the performance of the product process design and Ms = 538 – (361×0.42) – (39 × 0.72) – (19 × 1.74) – (39 × system in a short development cycle with reduced cost. This 0.78) (in °C) method combines the experimental and analytical concepts to Ms= 538 - 151.62 - 28.08 - 33.06 - 30.42 determine the most influential parameter on the result Ms= 294.82 response for the significant improvement in the overall performance. To solve this task, the Taguchi method uses a Choosing formula of Nehrenberg: special design of orthogonal array to study the entire process parameter space with a small number of experiments. Ms = 500 – (300 ×%C) – (33×%Mn) – (22×%Cr) – (17×%Ni) Preliminary tests without undergoing austempering process on – (11×%Si) – (11×%Mo) (in °C) samples using portable hardness tester EQUOTIP 3, Charpy Ms = 500 – (300× 0.42) – (33×0.72) – (22×0.78) – (17×1.74) test and tensile test via Houns-Field Tensometer gave – (11×0.28) – (11×0.22) (in °C) following results: Ms= 500 – 126 - 23.76 - 17.16 - 29.58 - 3.08 - 2.42 Hardness: 298 BHN Ms=298 Impact Toughness: 66 Joules Tensile Strength: 980 MPa Choosing formula of Steven and Hans: D) Orthogonal Array Ms = 561.1 – (473.9*%C) – (21.1*%Mn) – (16.7*%Ni) – The Taguchi method provides laying out the experimental (16.7*%Cr) (in °C) conditions using specially designed tables called Orthogonal Ms = 561.1 – (473.9 ×0.42) – (21.1×0.72) – (16.7×1.74) – Array (OA) [10]. An appropriate choice of orthogonal array (16.7×0.78) (in °C) depends upon the degrees of freedom. In our study 03 factors Ms= 561.1 - 199.04 - 15.192 - 29.058 austenizing temperature, austempering temperature and Ms=304.78

[ISSN: 2045-7057] www.ijmse.org 7 INTERNATIONAL JOURNAL OF MULTIDISCIPLINARY SCIENCES AND ENGINEERING, VOL. 5, NO. 10, OCTOBER 2014 soaking time is considered and 03 responses hardness, impact Graphical Representation: strength and tensile strength are measured. The three factors each consists of three levels i.e., Austenizing temperature The graphical representation of all three factors vs. the (830,850,870), austempering temperature (320,350,380) and responses are discussed in detail along with the trend it is soaking time (30,60,90) are considered. The Taguchi following: orthogonal array of L18(3×3) i.e., 03 factors with 03 levels is chosen for experimental runs. Graphical Representation for Hardness:

E) Austempering of AISI 4340 Two furnaces are used for austempering process. The samples are first heated to austenizing temperature in heat treatment furnace at temperatures of 830,850 and 870 oC in order to get austenite in microstructure of steel .the samples remain at these temperatures for one hour. The austenized AISI 4340 at mentioned temperatures are then quickly transferred to salt bath furnace with salt bath of nitrate-nitrate salt. In this research sodium nitrate + potassium nitrate in a mixture of 50% + 50% by weight is used. The samples are heated in salt bath furnace at austempering temperature range o o o of 320 C, 350 C and 380 C. The time of soaking is also other factor considered for our experimental; runs so these samples are dipped in salt bath at austempering temperatures for 30 Fig. 3: Hardness Vs Austenizing temperature min, 60 min and 90 minutes. The samples are finally taken out from salt bath furnace after its designed soaking time in The graph shown above represents the overall effect on salt bath and allowed to cool in open air. The austempered hardness of samples due to austenizing temperature during all samples are then tested for mechanical properties. experimental runs. With the increase in austenizing temperature hardness tends to improve. The graph shows that III. RESULT AND ANALYSIS the change brought by austenizing temperature is not so significant as the range of change is very small. The experimental runs comprising of eighteen set of experiments were conducted as per devised DOE by Taguchi method. The results obtained via each experiment were tabulated in the following format:

Table 1: Experimental Runs Data

Austenizing Austempering Soaking Hardness Impact Tensile Experiment Temperature Temperature Time strength strength

RUNS oC oC Minutes HRC Joules MPa 1. 830 320 30 373 79 1185 2. 830 320 60 377 82 1190 3. 850 350 60 392 80 1258 4. 850 320 90 376 88 1225 5. 870 320 90 382 86 1226 Fig. 4: Hardness Vs Austempering temperature 6. 850 350 90 384 86 1235 7. 870 350 60 392 78 1260 8. 870 350 90 382 84 1230 The graphical representation shows that improvement in o 9. 870 320 60 382 81 1227 hardness is observed till austempering temperature of 350 C 10. 850 320 60 379 83 1194 but with further increase in temperature the trend shows a 11. 870 380 30 378 79 1216 12. 870 380 60 390 79 1252 decline in hardness. The formation of lower bainite occurs o 13. 830 380 60 390 82 1250 from 300 to 350 C. During this time period martensite 14. 830 350 60 388 82 1245 microstructures are also present along with austenite which 15. 850 380 60 391 78 1255 imposes more hardness and overall lower bainite are much 16. 850 380 30 382 81 1228 17. 830 350 90 383 87 1232 harder than upper bainite. As austempering temperature o 18. 830 380 90 380 87 1212 increases from 350 C, upper bainite start forming which is lower in hardness properties than the lower bainitic structures.

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Austempering temperature has slightly significant effect upon strength as it shows slightly decrease than increase with increasing austempering temperature. As discussed previously upto temperature range of 350 oC due to austempering process lower bainite are formed in microstructure which are better in hardness but lower in impact strength. As temperature exceeds 350 oC upper bainite structures are obtained which are greater in impact toughness. Hence the impact strength decreases till 350 oC and an increase in impact toughness observed till 380 oC from 350 oC.

Fig. 5: Hardness Vs Soaking Time

The graphical representation shows here that soaking time has great influence upon the hardness of AISI 4340 samples. As time increses till one hour there is a direct relation regarding increase in hardness but as time is further extended to 90 minutes there is inverse relation observed with decrease in hardness properties. The soaking time of samples in salt bath gave necessary transformation of austenite into lower bainite and further upper bainite microstructures. The increase in hardness is due to presence of lower bainite and as formation of upper bainite takes place the hardness tends to decrese with time.

Fig. 8: Impact Strength Vs Soaking time

The graphical representation shows that soaking time has great effect upon impact strength. With increase in time of soaking in salt bath impact strength observed improved. Marked improvement in impact strength observed due to increase in soaking time. This is due to sufficient time provided to transform lower bainite to upper bainite completely as time period is enhanced.

Fig. 6: Impact Strength Vs Austenizing temperature

Austenizing temperature and impact strength has indirect relation with each other. The impact strength tends to slightly decrease with increase in austenizing temperature. The decrease in the impact strength is due to microstructure changes within samples of AISI 4340. However this decrease due to austenizing temperature is not so prominent as the range of change is very small.

Fig. 9: Tensile Strength Vs Austenizing temperature

The tensile strength is increased with austenizing temperature. A direct relationship is observed between tensile strength and austenizing temperature. The amount of increase is not so significant.

Fig. 7: Impact Strength Vs Austempering temperature

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S/NHB = -10 Log10 [mean of sum squares of reciprocal of measured data]

Table 2: Signal to Noise Ratio

Experimental S/N Ratio S/N ratio S/N ratio Runs Hardness Impact strength Tensile strength 1. 51.434 37.952 61.474 2. 51.526 38.276 61.1511 3. 51.865 38.062 61.199

Fig. 10: Tensile Strength Vs Austempering temperature 4. 51.503 38.889 61.176 5. 51.6413 38.689 61.177 Tensile strength graph shows an increase in strength with 6. 51.686 38.689 61.833 o austempering temperature till 350 C. But after that it shows 7. 51.865 37.842 62.007 little decrease with increase in austempering temperature. 8. 51.6412 38.485 61.798 Tensile strength is sometimes referred as directly proportional 9. 51.6413 38.169 61.776 to hardness. Same reason lies here for this case as was for 10. 51.5727 38.382 61.54 hardness. The formation of lower bainite till 350 oC cause increase in hardness and subsequently tensile strength and as 11. 51.549 37.953 61.698 the temperature increased further the formation of lower 12. 51.8212 37.953 61.952 bainite cause decrease in tensile strength. 13. 51.8212 38.276 61.938 14. 51.776 38.276 61.90338 15. 51.8435 37.842 61.9728 16. 51.6413 38.169 61.784 17. 51.664 38.7903 61.8122 18. 51.595 38.7903 61.67005

RESPONSE TABLES

Response Table for Hardness:

Table 3: Hardness response Table

Factors Signal to noise ratio averages Fig. 11: Tensile Strength Vs Soaking time Max-Min Rank Parameters Levels Level 1 Level 2 Level 3

Austenizin 830,850, 51.636 51.685 51.693 0.057 3 The tensile strength increase with soaking time but with g Temp 900 Austemperi 320,350, 51.7118 extended immersion in salt bath results in decrease in strength 51.5531 51.7495 0.1964 2 due to formation of upper bainite in its microstructure which ng Temp 380 6 Soaking posses good impact strength but lower hardness and tensile 30,60,90 51.5414 51.7479 51.6217 0.2056 1 Time properties.

Response Table for Impact Strength: IV. CALCULATIONS

Signal-to-Noise (S/N) Ratio: Table 4: Impact strength Response Table Signal-to-noise ratio where signal represents the desirable value and the noise represents the undesirable value [10]. Factors Signal to noise ratio averages Therefore, the S/N ratio consolidates several repetitions into Max-Min Rank one value, which reflects the amount of variation present. Parameters Levels Level 1 Level 2 Level 3 Austenizing 830,850 There are three S/N ratios available, higher is the better (HB) 38.3934 38.3388 38.1818 0.2116 3 Temp ,900 is used for all three responses found out i.e., hardness, Impact Austemperin 320,350 38.3928 38.3574 38.16388 0.229 2 toughness, and tensile strength. g Temp ,380 Soaking 30,60,9 38.02466 38.11977 38.7221 0.697 1 Time 0

Response Table for Tensile Strength

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VI. CONCLUSION AND SUMMARY Table 5: Tensile Strength Response Table Austempering process carried upon samples of AISI 4340 Factors Signal to noise ratio averages improved a lot overall mechanical properties of samples. Max- Rank Especially great improvement in impact strength was Min Parameters Levels Level 1 Level 2 Level 3 observed. Analysis of results showed that hardness was affected greatly by austempering temperature and soaking Austenizin 830,85 time, impact strength greatly affected by soaking time and g Temp 61.658 61.60 61.74 0.140 3 0,900 tensile properties affected greatly by austempering temperature. Overall the major variation in mechanical Austemperi 320,35 61.3824 61.7588 61.834 0.4516 1 Austempering process carried upon samples of AISI 4340 ng Temp 0,380 improved mechanical properties of samples to a great extent. Especially great improvement in impact strength was Soaking 30,60,9 61.6522 61.72 61.577 0.143 2 observed. Analysis of results showed that hardness was Time 0 affected greatly by austempering temperature and soaking time, impact strength greatly affected by soaking time and tensile properties affected greatly by austempering V. FINDINGS temperature.

The overall analysis of results shows following important ACKNOWLEDGMENT developments:  The highest reading for hardness was observed in Sincerest thanks to staff at the Material Testing Laboratory, experiment run No.7 in which hardness value of 392 Department of Manufacturing Engineering, NUST PNEC BHN was obtained. Karachi for their help and support throughout the project.  The highest value for impact strength was observed in Thanks to Mr. Javed Memon, Sales Officer at People Steel experiment No.4 in which impact strength of 88 Joules Mill Karachi for his support during procurement of materials was obtained. for the project and necessary guidance and help needed during research work.  The highest value for tensile strength was observed in experiment No. 7 in which tensile strength of 1260 MPa REFERENCES was obtained.  The overall best values observed in experiment run No.17 [1]. Hebsur M, Recent attempts of improving the mechanical with Hardness of 383 BHN, impact strength of 87 Joules properties of AISI 4340 steel by control of microstructure - A and tensile strength of 1232 MPa. brief review, Journal of Materials for Energy Systems. Vol. 4, Following percentage improvement in terms of Hardness, No. 1, pp 2837, 1982. [2]. Http://en.wikipedia.org/wiki/Austempering. impact strength and tensile strength of AISI 4340 steel [3]. Steel Heat Treatment Hand Book Second Edition. George E samples was observed keeping in view the initial values prior Totten. undergoing austempering heat treatment process and final [4]. Steels: Heat Treatment And Processing Principles. G.Krauss. values after they were austempered: nd [5]. Bainite in steels, 2 edition bhadeshia, h. K. D. H, (2001)

Table 6: Percent Improvement in Mechanical Properties [6]. Putatunda, s. K. (2003). Influence of austempering temperature on microstructure and fracture toughness of a high-carbon, high-silicon and high-manganese cast steel. Initial Final Values % Response Values Material and design , p. 435–443. Improvement [7]. Heat treatment : principles and techniques by t. V. Rajan, sharma c. P., ashok sharma Hardness 298 BHN 383 BHN 28.5 % [8]. Iron Making and Steel Making 1996, Vol. 23, No. 5 page. 433 [9]. P. Dhanapal Parameter Optimization Of Carbidic Impact Austempered Ductile Iron Using Taguchi Method, 66 Joules 87 Joules 31.81% strength International Journal of Engineering Science and Technol0gy Vol. 2(8), 2010, 3473- 3474. Tensile 980 MPa 1232 MPa 25.7% [10]. Design of Experiments Using the Taguchi Approach By strength Ranjit K. Roy.

The impact strength was improved up to 31.81 % which is indeed a great improvement for this grade of steel to be applied in aerospace industry where high impact strength is needed. Moreover along with impact strength, hardness and tensile strength properties are also improved to 28.5 % and 25.7 % which are also a good improvement.

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